High speed line printers use a multiplicity of print hammers actuated by electromagnets assembled in banks. The hammers are of necessity closely spaced. The requirement for compactness of the designs raises problems of overheating and magnetic interaction both of which can adversely affect the reliability of operation of the print hammers and the quality of the resultant printing. Current solutions for cooling the coils of the electromagnets and for eliminating magnetic interaction are thermally inefficient, complex to manufacture and require too much space thereby reducing the compactness and/or operating efficiency of the hammer units.